Racquet Restringing Payment issue

At our facility we require people to pay ahead of time on drop off, and not on pickup. We had a lot of issues in the past of people not paying, or refusing to pay. So with this in mind we haven't been able to use this currently implemented system. If anyone knows how to get payment upfront for the string job in court reserve, but not mark it as complete let me know. We just need a way of not having it be marked as complete, and automatically telling the client it is finished.